My invention pertains to power transfer apparatus for railroad track circuits. More specifically, the invention relates to apparatus which substitutes a track battery power supply to maintain track circuit operation when the normal alternating current power source fails.
Continuous operation of the track circuits in spite of power outages is a desirable condition in railway signaling. One specific situation relates to such track circuits used to control highway crossing warning signals where the failure of the track circuit from any cause, including power outage, activates the signals which warn and stop highway traffic. Such improper warning conditions are undesirable since they create a public attitude of ignoring the warning indication when valid. Track circuits energized with commercial alternating current are peculiarly sensitive to power outages. This is particularly true of the so-called Type C track circuits illustrated in the U.S. Pat. to O'Hagan, No. 1,914,958. These track circuits have an alternating current energy source and the track relay connected to the rails at the same end, usually at the highway crossing, and a rectifier means, normally a single diode for half-wave rectification, connected across the rails at the remote end of the circuit. Such track circuits are particularly useful in highway crossing warning systems where the track is otherwise unsignaled and wayside line circuits to the remote ends for power or indications are not available. A past practice has been to connect a standby track battery across the remote end of the track section in series or in parallel with the diode rectifier. More sophisticated arrangements connect the track battery to the rails only when an alternating current power outage is detected. This last arrangement requires a detection at the remote end of the presence and/or the absence of the alternating current energy in the rails. Connecting the track battery to the rails for permanent standby reduces the track circuit sensitivity and requires good ballast conditions and a large capacity AC source, that is, transformers with greater capacity. Similar conditions must be overcome when a changeover arrangement is used which substitutes the track battery at the remote end only when needed. It is also advisable and desirable to avoid any release of the track relay during a changeover period since this would cause at least momentary warning signals. Therefore, the requirements are for a Type C track circuit power transfer arrangement which allows the use of a lower energy AC source (smaller capacity transformers) with power regulation and which remains operable under poor ballast resistance conditions. Also no drop away of the track relay, i.e., false detection, should occur during the changeover to standby or in the reverse direction when AC power is restored.